Off-air Frequency Standard

This simple low cost instrument was built to confirm the calibration accuracy of a somewhat ancient Racal 9521 counter/timer, as well as that of the Philips Oscilloscope used for its development. The standard uses the carrier of the 198kHz Radio 4 AM transmitter at Droitwich UK. The circuit below uses a version of the once ubiquitous Ferranti ZN414 trf radio receiver chip, later cloned as the MK484. This 3-pin device appears to be still available in the UK from Bowood Electronics and possibly Rapid Electronics

198kHz frequency standard Although outwardly simple to use, the chip rewards attention to detail, particularly in the tank circuit where careful design of the coil for high Q is beneficial. The prototype used a surplus MW ferrite rod aerial from a pocket sized radio (wound with Litz wire), which was subtantially padded with a 1.5nF capacitor to bring it down to LW and within range of the "set in test" fixed trimming capacitors used. A fine tune was achieved by sliding the coil on the ferrite rod. Choice of Q1 and Q2 is not critical. Based on an original Ferranti design employing a 100mH choke on the ZN414 output to remove the superfluous modulation, this circuit performed well enough for the present purpose without this filtering, achieving about 1 part in 200,000 stability. Perhaps this hardly qualifies it as a "standard" compared to the Droitwich carrier's accuracy  better than 1 in 1010 but a useful piece of lab kit nevertheless. For those seeking the ultimate in this type of instrument, check out those at Quartzlock.

 off air frequency standard